Flywheel systems have been used for many years for storing energy in the systems, and then releasing that stored energy back into other systems. Flywheel systems provide a smoothing effect to the operation of internal combustion engines and other kinds of power equipment as well as in electrical applications such as uninterruptible power supplies, electric vehicles and battery replacement.
Various forms of high-speed energy storage flywheels using composite materials have been in use since the 1970""s. Many designs for these high-speed energy storage flywheels have included filament wound composite rings made of either glass or carbon fibers in an epoxy matrix. Such filament wound rings have the inherent advantage of very high hoop direction strengths, which are needed to match the very high hoop stresses generated during rotation. One drawback to the use of filament wound composite rings for the rim portion of a high-speed flywheel is inherently low radial strength resulting from absence of fiber reinforcement in that direction. Because the radial direction stresses in a filament wound ring being rotated are controlled by the non-dimensionalized radial thickness of the ring (ratio of ID to OD), such rings must be made thin. Because a single ring must be made very thin so that it does not fail at a prematurely low rotational speed, the ring becomes less effective for energy storage. Another problem that arises is that the hub, which is used to attach the rim to the shaft, must be made larger due to the larger ID of the filament wound ring. This causes unacceptably high stresses in the hub which reduces the maximum speed possible and hence energy storage capability of the flywheel.
To avoid the problem of excessive radial stresses in the filament wound ring without requiring the ring to be made radially thin, multiple thin rings can be placed in a concentric arrangement, which will then function as one thick ring. One way to couple together several radially thin rings to make a thicker flywheel rim is by press-fitting. By assembling the rings together with a radial interference between each ring, the rings can be driven into radial compression at zero speed. When the rotor is spun to high speed, the radial compression between the rings is lessened. At failure speed, the pressure between two or more rings goes into tension and the rings separate.
Although press-fit rims have been employed in several flywheels designed to date, there is a need for an optimal design of a flywheel that employs press-fit composite rings. Such optimization generally applies to simultaneous consideration of rotor performance and the cost of manufacture. To date, experimental rims have been assembled from as many as ten rings and as few as two. A wide variety of fibers have been used in the composite rings and the ratio of ID to OD has been widely varied.
Accordingly, this invention provides a cost-performance optimized flywheel rotor assembly having a flywheel rim comprised of press-fit composite rings.
The composite flywheel rotor of this invention includes a flywheel hub having a tapered outer surface and a concentric flywheel rim. The flywheel rim has multiple rings axially press-fit together to precompress the rings to form a composite flywheel rim. Each ring is made of approximately equal radial thickness and the entire non-dimensionalized radial thickness ratio of the assembled rim should be between approximately 0.38 to 0.48. The rim is optimally made up of four or five individual rings, each of which rings has tapered inner and outer diameters, preferably tapered at small angles to produce large radial forces when the rings are pressed onto each other and the hub by pressing axially, resulting in a high radial compressive preload in the assembled rim. A taper angle of 1-5xc2x0 is suitable. The assembly uses standard modulus (30-40 Msi) or intermediate modulus (40-50 Msi) carbon fiber for all of the rings.